


something tells me you're the devil's daughter

by Anonymous



Category: IDOLiSH7 (Video Game)
Genre: Aphrodisiacs, Drink Spiking, Dubious Consent, Extremely Dubious Consent, F/F, Manhandling, Rule 63, Sibling Incest, Spanking, Vaginal Fingering
Language: English
Status: Completed
Published: 2019-12-10
Updated: 2019-12-10
Packaged: 2021-02-26 07:41:38
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 2,929
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21749953
Author URL: https://archiveofourown.org/
Summary: Seto gets a kick out of humiliating her sister.
Relationships: Seto Ranvalt von Northmare/Rokuya Nagi
Comments: 2
Kudos: 18
Collections: Anonymous





	something tells me you're the devil's daughter

**Author's Note:**

> PLEASE READ THE TAGS. PLEASE READ THE TAGS. P L E A S E R E A D T H E T A G S.
> 
> If you read the tags and you're still here, enjoy!!

Seto had always enjoyed messing with her sister. She had it down to an art - set it up carefully, so nothing seemed particularly amiss, and then before she knew what was happening, Nagi would end up dunked head-first in a situation she didn't know how to deal with.

Case in point: that time when they were younger, and Seto had made Nagi act as tour guide for ambassadors of some country or another, the language of which Nagi was still new to. Watching her stumble over her words, mess up her usual practiced, charming demeanour, cracked Seto up no end.

And then, more recently, when Nagi had expressed a desire to learn how to cook - Seto had sent her down to the kitchens at dusk to "familiarise herself with the equipment", then snuck down after her, jamming the door from the outside so it appeared the lock was broken. She'd had to cover her mouth to stifle her laughter as she listened to Nagi wail, calling for help - and then, later, trying to appease any ghosts that might be wandering in the dark. It had been hilarious… But also, the sound of Nagi crying, begging for help, awoke something new in her. A hot, curling sensation in the pit of her belly, making her want to close her eyes and imagine them both somewhere else. In her bed. With Nagi helpless to refuse…

Seto swallowed thickly, making sure the corridor was clear before slipping her hand beneath her skirts, rubbing her crotch clumsily through her tights. Even through the layers of undergarments, her fingers came away wet and sticky.

She shuddered.

She wanted more.

* * *

After a few months, Seto had all but forgotten about her desire to make Nagi squirm… That was, until Nagi had come to her, determined, and announced that she would be moving to Japan imminently.

Seto knew that Nagi was nineteen now, and capable of making her own decisions - and if she brought Thorvald with her, at least initially, she  _ might _ not waste away because she had no clue how to take care of herself. But still, the thought of it made Seto's blood boil. She couldn't let her leave. She couldn't believe Nagi would even consider it, leaving her, leaving her  _ kingdom _ …

Seto was suddenly overcome with the need to make her pay. Humiliate her, as punishment for leaving them all - leaving  _ Seto _ \- behind.

* * *

"It's not long until you leave now, is it?"

"No, Sister. In three days."

Seto knew that, of course. She'd been counting down the days ever since Nagi had announced her intentions.

"Then come, Nagi. Let's drink together in my room. We might not get another chance for a very long time, after all."

Nagi hesitated for a split second, then smiled.

"Alright, Sister. Do you have that champagne I like?" 

"Of course. Only the best for my darling Nagi."

* * *

It really was too easy. All Seto had to do was crack the tiny vial she'd ordered, tipping its contents into Nagi's glass while her back was turned. Nagi was none the wiser as she turned back, picking up her glass and taking a long sip. Seto watched the way her elegant fingers curled around the stem of the glass, a brief shudder running through her as she imagined herself holding that hand, pushing those fingers against her own core.

_ Maybe it's not just revenge you want _ , she thought. She chased that thought away as quickly as it came to her.

"So, Nagi… How long do you plan to stay there, exactly?" 

"As long as it takes. I have to find my friend, and I have an errand to run besides. Plus, I need as long as possible to truly appreciate my spiritual home of  _ Akihabara _ …  _ Cocona  _ is waiting for me."

"I see," Seto replied, forcing a smile. She needed to change the subject, or Nagi would be talking about that damned cartoon until sunrise, drugged or not. "But won't you miss us all? Father, Thorvald… Me?"

Nagi fell quiet.

"...Of course I'll miss Father. Thorvald will be with me for the first few weeks anyway, but we've never been too close to begin with…"

"And me?" Seto pushed.

"...And… I'll miss you," Nagi replied. Seto noticed her stiffness, and she held back her own sneer.

_ How dare she. After all I've done for her.  _

"Really?" 

"Of course…"

"You don't sound too sure."

"I'm sure…!" 

"Then prove it."

Seto was taken aback by her own words, blinking as confusion settled on Nagi's face. Damn. She'd almost moved things too fast… Everything had to go exactly according to plan. She had to backtrack. 

"...No. No, I'm sorry, Nagi. I don't know what came over me."

"Sister…"

Seto watched Nagi's expression soften before she sighed, finishing off her glass. She put the glass aside, then to Seto's surprise, gently took Seto's hand in hers.

"Nagi…"

"Sister, you seem… Distracted. Is everything okay?"

Seto looked like a deer caught in headlights, caught off-guard by Nagi's sudden softness. This… Was new. Or was it just  _ old _ ? Had it been so long since they'd been like this that she'd just forgotten? Her face felt hot as she swallowed.

None of that mattered, anyway. It wouldn't be long now.

"I'm fine, Nagi. I'm sorry for worrying you. I suppose… I'll just miss you dearly."

"Sister… I-  _ Oh… _ "

Seto noticed the change almost immediately. 

One moment, Nagi had been looking at her with kind, soft eyes… And the next, she was pulling her hands away like she'd been burnt, her face suddenly red, expression taut.

_ Her cute little Nagi. _

"E- Excuse me, I…" Nagi stumbled to her feet, heading towards the door. "I must… Take my leave…"

Seto hid her smirk behind her hand as she watched Nagi try to open the door with shaking hands. Her attempts grew more frantic as she visibly pressed her legs together, knees buckling…

Seto would have to make sure she gave Thorvald a bonus for blocking the door from the outside.

"S- Sister, the door…!" Nagi whimpered, still not giving up as she rattled the handle. Seto resisted the urge to slip her hand between her own legs already… This was just too delicious. 

"Oh? What's wrong, Nagi?" 

"The door won't open, and I… I…!" 

"What is it? Do you need the bathroom?" 

"No!" Nagi yelped, somehow even redder than before. 

"Then… Are you unwell?"

"I… Yes... I need to leave..."

"Oh, dear." Seto finally stood up, following Nagi over to the door. She gave the handle a token shake herself, then sighed. "It does seem as though it's broken. What's wrong exactly, Nagi? A fever?" 

She placed her hand on Nagi's forehead, only for Nagi to shiver, a tiny, near-inaudible gasp slipping past her lips. She really was warm - but Seto knew the cause. Unable to resist, her hand moved to cup Nagi's cheek, and her thumb swiped out, just brushing Nagi's bottom lip.

The reaction was more substantial this time, with Nagi's legs shaking, her lips parting with a soft whine.

"Ah…"

"Nagi?"

"S- Sister… I need…" 

"Shh… I understand. But, if you want… I could help you. I promise, I won't tell a soul."

Nagi's eyes widened, and to Seto's annoyance, she pulled back, if only by a fraction of an inch.

"You… You don't… I think you misunderstand…"

"I understand perfectly," Seto replied, growing irritated as she leaned in close so her breath tickled Nagi's lips. Nagi whimpered again, but this time, she made no attempt to pull away. "I can help. We're stuck in here for now, and you need relief. Who would I be to leave my sweet little Nagi to suffer?"

Nagi remained quiet for a moment. Then, she opened her mouth, only to close it again - Seto just smiled, revelling in the thrill of seeing her sister so helpless.

"But…" 

"Shh," Seto whispered, leaning in closer. "Don't worry. I'll make you feel good, Nagi."

Nagi didn't pull away as Seto slowly closed the gap between them, pressing their lips together in a tentative kiss. To Seto's surprise, Nagi relaxed almost instantly, lips parting with a soft whine. 

They settled into an easy rhythm, Seto chasing Nagi's lips whenever she tried to pull back. She was pleased to find Nagi doing the same when she pulled away briefly for air, the knowledge that Nagi needed her,  _ wanted her _ , sending heat flooding to her groin. Eventually, she did pull away, cupping Nagi's cheek affectionately.

"So cute… You taste so sweet, Nagi. I wonder if the rest of you tastes just as delicious…" 

To illustrate her point, Seto traced her fingers down Nagi's side, eliciting a shiver in response.

"S- Sister…"

Seto hushed her, slipping her cold fingers under the hem of Nagi's blouse.

"Nagi… Tell me. Do you want me?" 

Nagi remained silent for a moment, visibly swallowing as she thought. 

"...Yes."

"Yes what?" 

"Yes… Sister. Help me. Please. I- I can't, I need…"

Seto smiled devilishly, fingers moving up to deftly undo Nagi's nightshirt. She was already so wet just from the thrill of seeing Nagi's soft, pale breasts before her, nipples hard with arousal…

"Here. Does this feel good, Nagi? Letting me see you like this? Touch you like this?"

She drank in Nagi's mewls and soft cries as she lightly ran her fingers over her nipples, only to pinch them harshly. Nagi squirmed against her, not so subtly grinding her legs together, desperate for friction. It was somehow both too much and not enough… 

"Nagi…"

"M- Mmn…"

"I bet you're so wet, aren't you? So desperate for your own sister to touch you…"

Nagi's reaction was immediate, eyes widening and lips parting as if she'd been caught. Her cheeks reddened as the humiliation set in, and Seto had to restrain herself from launching herself on her right there and then. Instead, she sat back down on her bed, pulling Nagi down with her.

"I…"

"It's true, isn't it? My cute little Nagi… What are you thinking about?"

Seto watched, amused, as Nagi refused to answer, averting her gaze. 

"Nagi."

"I-  _ ah,"  _ Nagi gasped, as Seto gave her nipple a rough tweak, "I… I won't say. This… This is wrong, it's…"

Seto just snorted, shoving her hand under Nagi's skirt without ceremony and pressing her fingers forcefully to Nagi's folds through damp cotton.

"And yet you're so wet… You really want this, don't you? Want me to fuck you? Do you want me to stick my fingers in you, rub your clit, fuck you with my tongue?" 

Nagi shrieked at the sudden contact, thrashing but bucking her hips all the same, pressing harder against Seto's fingers on instinct. Her sister's words filled her head, making her practically delirious with need and embarrassment.

"I…"

"Look at you. You don't even need to say it… God, you're filthy. Who would've thought?" 

Nagi's breathing was ragged, her body a live wire in Seto's hands. Seto moved her fingers in a figure eight, while still kneading Nagi's breast with her other hand. It sounded so good… Nagi's disjointed breathing, mixed with the sound of Seto's fingers working against her wetness, would be burned into her memory for years to come.

"Sister… I need…" 

"What do you need, Nagi?"

"I - I don't know, I… Just do something…"

Seto moved suddenly, flipping Nagi so she was lying on her front. She pressed her down firmly against the bed, trapping her, while pulling her skirt up, exposing the white cotton of Nagi's panties, now translucent with arousal. 

"I'd appreciate it if you didn't order me around, Nagi," she said simply, cool fingers moving to cup Nagi's behind, massaging soft skin. Nagi let out a weak cry in response. "I'm helping you, remember. I've always helped you. You don't get to tell me what to do." 

With that, Seto raised her hand, then brought it down in a sharp  _ smack _ . Nagi's knees buckled as she let out a loud, broken moan.

"You don't get to turn away from me, after everything I've done." 

_ Smack.  _

"You don't get to leave me." 

_ Smack.  _

"If you want me, beg."

Rather than spanking her again, Seto went back to massaging Nagi's behind again, her now-reddened skin a stark contrast to Seto's pale fingers. Nagi's breathing was still laboured as she buried her face in Seto's blankets, hips twitching with need.

"...Please…"

"I can't hear you," Seto replied, digging her nails in.

" _ Ah--  _ please…!!"

"Please what? Uncover your face, Nagi. I can't hear a word you're saying."

Weakly, Nagi looked up, and Seto felt another rush of heat flow through her as she noticed the wetness left behind on her blanket. Was it drool? Tears? She didn't know. Either would be good. Nagi's face was adorably flushed as she tried to avoid Seto's gaze.

"Please… Touch me. Touch me  _ there… _ "

Seto was half tempted to keep this going, ask her  _ where _ , but she was starting to grow impatient herself. Instead, she took pity, hands moving to the waistband of Nagi's panties.

"Good girl."

Seto was unable to suppress a shudder as she pulled them down, exposing Nagi's perfect, pink lips. She wanted to touch. God, she wanted to  _ taste.  _ Nagi was shaking like a leaf as Seto knelt beside her, leaning in close so she could whisper in her ear. 

"Nagi… My, you're pretty. So pretty… And you're all mine, aren't you?" 

She didn't give Nagi a chance to respond before she thrust two fingers in, leaving Nagi choking on a sob. Squelching filled the room as Seto fucked her roughly, Nagi's wetness coating her hand.

"You sound so good… You really want this, don't you? Want me to fuck you?"

"Y… Yes," Nagi sobbed, pressing back onto Seto's fingers. Seto swallowed, unconsciously grinding down against her own heel as she added a third. Nagi took her in easily, making her distantly wonder if Nagi could take her whole fist.

"Have you thought about this before?"

" _ Ngh _ ," Nagi said by way of a response, burying her face in the covers again. Seto removed her fingers, eliciting a distressed cry from her sister.

"What have I told you about that?"

Without warning, Seto grabbed Nagi by the waist, flipping her over again so she was on her back. Now, she had nowhere to hide.

"Now tell me. What did you say?"

Nagi visibly swallowed, hands twitching as if she wanted to cover her face, but thought better of it.

"...Yes. I have… I have thought about it."

Seto wasn't entirely sure she wasn't dreaming, now. 

"You… Have. What have you thought about?" 

Nagi looked as if she wanted the ground to swallow her up, but that only served to make Seto wetter. 

"I… This. You touching me… Making me taste you."

_ Oh, dear lord in heaven,  _ Seto thought.  _ This isn't real. It  _ can't _ be.  _

Deciding Nagi's honesty deserved a reward nonetheless, Seto lightly ran her index finger over Nagi's clit. Nagi whined in response, back arching. Seto couldn't get enough of this… Nagi, bare and naked before her, face red and flushed, her sweet, soft breasts exposed, her own fingers working against Nagi's sopping wet cunt…

And then, it hit her.

"Nagi. Has anyone else ever touched you like this before?"

Now, Nagi couldn't stop herself from covering her face in embarrassment. Seto couldn't hold back a laugh of sheer delight. 

"...No."

"So your first fuck is your sister. How shameful…"

In contrast to her words, Seto increased her speed, abusing Nagi's clit until she knew she was close. She wanted so badly to use her mouth, taste Nagi's wetness, make her come using just her tongue… But, she reassured herself, this was just the beginning. If Nagi truly wanted this, then…

" _ Fuck!"  _ Nagi cried, the uncharacteristically foul word falling from her tongue making Seto shudder with delight. She sped up as much as she was able, despite the growing ache in her wrist.

"Come for me, Nagi. Let me see you. I bet you look so cute when you--" 

Seto was interrupted as Nagi let out a strangled cry, body spasming and face contorting with pleasure. Seto felt dizzy as she felt fluid gush around her hand, soaking the sheets beneath them. She didn't let up until Nagi twitched with overstimulation, at which point she removed her hand and leaned in, sealing Nagi's parted, abused lips with a fierce kiss, stroking her soft hair in a gesture that could almost be seen as loving.

"I was right," she said, once their lips had parted. "You did look cute… As expected of my darling little sister."

Nagi didn't even have the energy to be embarrassed. Instead, she just smiled gently, huffing out a weak laugh.

"Sister… Thank you. I… I Feel much better now."

"I'm glad," Seto replied. "But I'm afraid there's a problem. You may feel better, but now… Well."

In lieu of an explanation, Seto stood up, and began carefully stripping off her clothes. Nagi watched - in what seemed like awe - as she removed her blouse, skirt, and undergarments, leaving them all in a neat pile on her chair. Seto returned to the bed, lying beside Nagi and taking her hand… And then, just as she'd imagined before, moved it towards her, pressing Nagi's fingers between her wet folds. She took a sharp breath at the contact, and had to hold herself back as she saw the redness on Nagi's cheeks flare back to life, her mouth agape. 

"...You see how it is. Won't you be a good girl and help your sister out, Nagi?"

As it turned out, she was more than willing.

**Author's Note:**

> THANK YOU FOR READING!!
> 
> This was a commission for a kind donor who also wishes to remain anonymous. I love you, anonymous donor. I do. Thank you for giving me a good reason to feed myself, your prompt was so my brand I wanted to scream


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